It's hard to critique an idea that's literally nothing but a brief blurb and a handful of titles.

(Also, as dankaar said, this would be difficult to pull off without shattering suspension of disbelief. A massive multi-breach definitely has story potential, but you'll need to put some thought into how the Foundation might've screwed up hard enough to let it happen.)

I'm now imagining a series in vein of "And then I died" of short stories from the perspective of Foundation staff or GoI intruders trying to release dangerous SCPs from containment (being first-person it would include their motivation and reasoning for wanting to release it to begin with) and then how they get killed by it due to it not behaving exactly as they anticipated/oversight on their part/etc.

Here's the deal: while there's a few Keters that are reasonably "easy" to recontain (you know, if they are containable), the general reaction to not small amount of them would be "nuke the site, hope that does the trick, prepare for the worst if that failed".

That, and the fact that many Keters nowadays have containments that are considerably more esoteric than just "keep it in a cell", so a breach in the traditional sense is more unlikely.

If you need some Deus Ex, there's always the Apollyon(not going to post that massive spoiler in case you haven't read it yet). Also, there are several Keter capable of releasing other Keter in a chain reaction. The Star that Hates comes to mind. The Star is implied to have access to almost ALL of the Foundations most sensitive files, up to and including the identities of the O5 and all secure sites. It could easily leak said information to M,C & D, The Church of the Broken God or GOC. Then there's the bigfoots. 'Given choice for now, not forever'. They are highly intelligent, with powerful biotech. They might be intentionally be releasing Keters that will cause massive destruction without outright obliterating the world. Finally, the Red Pool normally releases monsters of the 'brute' variety, but what if it released a smart one instead?